Powder metallurgy process

ABSTRACT

A METHOD OF PRODUCING AN IMPROVED TOOL STEEL HAVING A SUPERIOR ALLOY HOMOGENEITY WHICH COMPRISES THE STEPS OF PROVIDING A CHARGE OF IRREGUALRLY-SHAPED, FRACTURED, PREALLOYED TOOL STEEL POWDER OF A HOMOGENEOUS COMPOSITION, EXPOSING THE POWDERED PARTICLES TO A HYDROGEN REDUCING ATMOSPHERE TO REDUCE THE ODIXE CONTENT AND SLOWLY COOLING THE REDUCED POWDER PARTICLES TO AVOID IMPARTING AN OBJECTIONABLE DEGREE OF HARDNESS WHICH WOULD INTERFERE WITH SUBSEQUENT COLD PRESSING OF REDUCED POWDER PARTICLES, COLD PRESSING THE REDUCED POWDER PARTICLES TO A BILLET FORM HAVING A GREEN STRENGTH ADEQUATE TO MAINTAIN STRUCTURAL INTEGRITY UPON FURTHER HANDLING OF THE BILLET, AND SINTERING THE COLD PRESSED BILLET TO IMPART A THEORETICAL DENSITY OF AT LEAST 85% TO THE TOOL STEEL WITH THE SINTERING BEING CARRIED ON UNDER CONDITIONS TO MINIMIZE GRAIN GROWTH AND PHASE SEPARATION OF A DEGREE INSENSITIVE TO SUBSEQUENT PROCESSING.

United States Patent Int. Cl. B22f 1/00 US. Cl. 75-213 35 Claims ABSTRACT OF THE DISCLOSURE A method of producing an improved tool steel having a superior alloy homogeneity which comprises the steps of providing a charge of irregularly-shaped, fractured, prealloyed tool steel powder of a homogeneous composition, exposing the powdered particles to a hydrogen reducing atmosphere to reduce the oxide content and slowly cooling the reduced powder particles to avoid imparting an objectionable degree of hardness which would interfere with subsequent cold pressing of reduced powder particles, cold pressing the reduced powder particles to a billet form having a green strength adequate to maintain structural integrity upon further handling of the billet, and sintering the cold pressed billet to impart a theoretical density of at least 85% to the tool steel with the sintering being carried on under conditions to minimize grain growth and phase separation of a degree insensitive to subsequent processing.

This application is a continuation-in-part of application Ser. No. 742,194, filed July 3, 1968, which is a continuation-in-part of the earlier application Ser. No. 657,087, filed July 31, 1967, both abandoned.

This invention relates to an improved method of making dense alloy steel, and has for an object to provide such a method which affords greater assurance of producing a homogeneous, dense alloy steel than is afforded by prior known methods, and which produces such alloy steel more economically than can be done by prior known methods.

A related object is to provide a method capable of making diiferent and superior steel alloys as compared with those which can be made by prior known methods.

A further related object is to avoid or reduce waste of starting material.

Another object is to form cohesive billets or compacts of the powdered alloy steel by a cold press, thereby avoiding the necessity of forming the billets by the less economical hot press which has heretofore been proposed.

Conventionally, tool steel stock is cast from an alloy and hot worked to final shape. Typically, the process wastes up to 60% of the cast material. Severe alloy segregation resulting in a heterogeneous structure is inherent in the casting procedure and extensive material hot working and machining is required to produce an acceptable homogeneous raw stock. Hot working and the machining away of segregated material both serves to homogenize the composition and microstructure of the ingots.

Powder metallurgical techniques have also been considered heretofore for forming tool steels. This approach has had only limited acceptance since it has not proven to be either fully technically or economically sound. The starting material has generally been atomized particles or pre-alloyed tool steel, which are essentially spherical in shape. It has not heretofore been demonstrated practical to cold press such atomized particles to form a billet sufficiently cohesive for further necessary processing.

3,744,993 Patented July 10, 1973 This difficulty in forming a billet with adequate green strength to maintain structural integrity has heretofore directed efforts to adapt powder metallurgical techniques for forming tool steels to a hot press operation. Hot pressing techniques employ an elabtorate pressing apparatus which is generally capable of processing only a single part at a time.

Recognition has heretofore been given to the fact that soft metals such as iron and low carbon steels can be molded from metal powders by cold-pressing under high pressure to cold-weld the metal particles, with this being followed by sintering. This is well-stated in US. Pat. 2,827,407," Carlson et al., issued Mar. 18, 1958 (British counterpart 797,287). That patent further explains that it has been proposed to produce articles from high alloy steels such as high chromium and high chromium-nickel steels, that is, the stainless steels, by using pre-alloy powders of the desired composition, including the requisite amount of carbon, by pressing the powder followed by sintering, but that because of the poor compressibility of such powders it has not been possible to produce compacts of high density, that is, above of the theoretical density, even if the compacts are coined and re-sintered. While noting that high density in such alloys is needed for good fatigue resistance, that patent goes on to state that the art of powder molding has long sought a process which would be capable of producing dense heat treatable articles of stainless steels having the fatigue resistance and other physical properties of a wrought article of the same composition.

That patent proposes to overcome the inability of the art prior thereto to make a satisfactory dense alloy steel from powder by using powder which has not been prealloyed. Specifically, the process of the patent involves mixing powdered iron of high compressibility and powdered alloying ingredients and compacting this mixture of tin-alloyed powders, and then sintering the compact. It asserts that this process can make a steel article containing between 11% and 25% chromium and up to 2.5% nickel, with a density between 96% and 98% of theoretical. That patent discloses that if alloyed powders are used instead of un-alloyed powders, the powders do not readily weld together on cold pressing, and the compacts formed from them have relatively low density.

Although the process of the Carlson et al. patent represents an advance over prior attempts to use powders, inasmuch as it does not use pre-alloyed powders it does lack the assurance of a homogeneous composition which is inherently characteristic of a pre-alloyed powder compact.

In accordance with the present invention there is provided a novel powder metallurgical process utilizing prealloyed steel powder and cold pressing, which eliminates the requirement for hot pressing the powder in order to form a satisfactory billet or compact. Not only does this discovery simplify and reduce the cost of making known steel alloys, but it also permits the making of novel tool steel alloys which have not heretofore been attainable because of their excessive segregation characteristic when attempting to cast them, and also because of their inability to be hot worked to achieve a homogeneous structure. The powder metallurgical process of the instant invention in eliminating the requirement for hot pressing provides a significant advancement and for the first time a practical approach for the widespread adoption of powder metallurgical techniques in the production of tool steel.

The process of the present invention employing special powder metallurgical techniques permits the manufacture of tool steel stock with little or no waste of starting material. The amount and severity of hot working are significantly less than that normally required for the production of tool steel by the commonly-employed casting procedure. In one preferred embodiment of the process of the invention the starting material constitutes tool steel scraps such as machine turnings which may be successfully employed to produce tool steel which is at least equivalent and sometimes superior to that originally made. 1

The novel process of the present invention employs as starting material irregularly-shaped, fractured, pre-alloyed tool steel powder particles of a homogeneous composition. The expression irregularly-shaped as used herein means that a substantial proportion of the powder particles have outer surfaces or substantial parts of outer surfaces which are nonspherical and without smooth or uniformly changing curvatures and contain angular edges. The expression pre-alloyed as used herein means that the steel from which the powder is made contains other material than iron, such as, for example, molybdenum, chromium, tungsten, vanadium, etc. and carbon. The term homogeneous as used herein means that all the powder particles have substantially the same alloy composition and that the proportion of the ingredients constituting the alloy is the same throughout the material forming each powder particle.

Such irregularly-shaped powder particles are conveniently prepared with impact pulverization. The impact pulverization can be performed, normally after degreasing, upon, for example, alloyed tool steel scrap such as machine turnings, atomized particles or water shot particles. The impact pulverization can, for example, be in accord with that disclosed in U.S. Pat. No. 3,184,169, which is better known commercially as Cold Stream process. In that process, alloy particles are bombarded against a target to pulverize them. Other means of impact pulverization such as bombarding of pre-alloyed particles against each other, utilizing suitable apparatus, are also available. Although the many particles of the powder thus pulverized differ from each other in size and shape such that no precise physical description can be given for all the individual particles, the physical nature of the powder is accurately described as that resulting from the process of impact pulverization as just described. The term impact pulverization as used herein means the process of bombarding alloy particles against some surface or against one another.

The powder resulting from the impact pulverization is then cleaned in a hydrogen atmosphere to rid it of most of the oxides. Carbon may be added to assist in the removal of oxides in the cleaning operation. The carbon, if employed, serves primarily to balance decarburization during hydrogen reduction and sintering and thus maintain a desired carbon content of the tool steel. Carbonaceous materials other than carbon may be used, such as an organic solution, which will supply carbon in an equivalent weight percent to pure carbon. Suitable carbonaceous materials include sugar in ethyl alcohol solution and hydrocarbons, such as benzene. After the material has been cleaned it is cold pressed at ambient or near ambient temperatures into a billet form which may take various forms including rods, blanks of various shapes and sizes, etc. Hydrostatic pressing is advantageously used for this operation. In one embodiment of the process of the invention, cold pressing is achieved by explosive compaction. Sufiicient pressure is employed in the cold pressing operation to impart a green strength to the pressed powder article which is adequate to maintain structural integrity upon further handling of the billet. In order to accomplish a successful cold pressing of the powder, it is important that in the hydrogen reduction of the material preceding cold pressing that the reduced powder particles be slowly cooled to avoid imparting an objectionable degree of hardness which would interfere with the subsequent cold pressing. Generally speaking, the reduced powder should have a Rockwell C (R hardness of not more than about 55 for 100,000 p.s.i.

pressing pressure, 30 for 60,000 p.s.i. If cooling is achieved too fast the powder particles will harden to around R 60 and will be nearly impossible to press even at an elevated pressure of 100,000 p.s.i. Preferably, the hydrogen reduced powder is cooled at less than 300 F. per hour. In one embodiment of the process the green billet is then transferred to an atmosphere-controlled furnace wherein a cold pressed billet is sintered to impart a theoretical density of at least Typically, the cold pressed billet during furnace sintering is heated to a temperature within the range of about 2150 F. to about 2350 F. to obtain the desired theoretical density.

Either mechanical or isostatic pressing at about 60,000 to 100,000 p.s.i. and higher is generally capable of consolidating tool steel powders up to 86% of theoretical density. The compacts are then usually either sintered to a desired density, or sintered and worked to the desired density. The latter approach of sintering and working is preferred since it minimizes the severity of thermal treatment and helps hold down the grain size of the material. If in the instance of explosively compacted powder a still higher density is desired, this may be achieved through a furnace sintering at a temperature typically in the range of about 2150 F. to 2350 F. or a nonfurnaced sintering employing hot rolling. Sinter rolling is desirably accomplished in the temperature range of 2000 F. to 2200 With the foregoing rolling temperatures, sintering is actually fostered during the hot working operation.

The method of the invention lends itself readily to mass production techniques, wherein several pieces may be pressed and subsequently sintered in succession without tying up large complicated machinery on a per piece basis as is required in hot pressing. The pressed particles are then as indicated placed typically in a sintering oven. Under certain conditions and for some tool steel uses, no further work is required. However, in other instances, the sintered product is then hot deformed, primarily for densification purposes. The hot deformation is not an extremely severe working of the product as compared to prior art techniques. The hot worked or sintered product is then conventionally heat treated. As will be shown in more detail, the resultant tool steel product made in accordance with the process of the invention, frequently possesses unique properties heretofore unobtainable in tool steels of like carbon content. The inventionwill be more fully understood from the following detailed description and specific examples set forth.

IMPACT PULVERIZATION A particularly effective means for achieving impact pulverization to produce the fractured powder particles is disclosed in the aforementioned Pat. No. 3,184,169 pertaining to the so-called Cold Stream process. Utilizing for example that process, one may start with scrap machine turnings or chips of tool steel alloy and subject them to the pulverization disclosed. Water shot formed of scrap material may also be used in the Cold Stream process. Crude atomized particles are particularly suit able for impact pulverization. Both the crudely atomized particles and the water shot may be formed directly from newly melted alloy where it is desired to eliminate the waste of material inherent in casting large ingots and the extreme working of such ingots required in prior art. The impact pulverization is desirably carried out to produce a preferred particle fraction of 325 mesh powder although particle fractions of mesh have been used but less effectively. The average particle size of the powder is desirably in the range of 2 to 60 microns, and preferably in the more limited range of 6 to 14 microns. The smaller size particles are more easily pressed and sintered to obtain the desired density. It has been found that the fractured particles obtained through impact pulverization are irregularly-shaped, unlike atomized particles in this respect, and because of this when pressed successfully interlace and interlock with one another. Furthermore, unlike atomized particles which have a hard surface resulting from the atomized operation, the fractured particles produced by impact pulverization possess outer surfaces no different from the remaining inner structure, thus making the fractured particles particularly susceptible to cold pressing. The use of the fractured particles obtained by impact pulverization provides a starting material which is susceptible to relatively easy processing conditions, thereby making powder metallurgical techniques to form tool steel economically feasible.

Particle size distribution has proved to be an indication of a powders ability to obtain the density indicated by its average particle size and sintering cycle. The most desirable powder size distribution represents a Gaussian ratio whose curve preferably has a general bell shape. It is known from a number of distribution analyses that if the powder contains a too large a proportion of small diameter particles, that is, the distribution curve is misshaped to include too large a proportion of small particles, there is a tendency for the tool steel product to have areas of high density. On the other hand, if the distribution curve tends to include a too large proportion of large size particles, there is a tendency for the tool steel product to have areas of porosity that are difficult to sinter closed. The desirable Gaussian distribution facilitates the production of the improved tool steel of the invention and to the extent that the distribution strays from a general Gaussian ratio, the conditions of cold pressing and sintering become more severe.

CLEANING After the powder comes out of the impact pulverization process, a cleaning operation is generally required. The materials typically have from .5 to 8% oxides present as an impurity. The impurity presents several drawbacks to producing a suitable end tool steel product. Firstly, one cannot get a dense structure since the low density oxide forms such a significant percentage of the grain boundaries. Because the oxides have low strength and are brittle, the composite alloy incorporating too much oxides has little toughness. In order to effect cleaning, the powder is hydrogen reduced, preferably at around 1800 F. for low and above 1800 F. However, at 1600 F. reduction is slow while at 1900 F. the material tends to sinter. A preferred range for oxide removal is 1750 F. to 1875 F. The time duration is not too critical as long as the whole about one hour. The range of temperature may extend bebatch of powder gets up to the temperature and enough time is allowed to substantially rid the material of the oxides. Hydrogen reduction will typically lower the oxide content to .3 to .25 The remaining oxides may be sub stantially removed through the incorporation of a small amount of carbon in the powder. Thus, by adding from 0.1 to 1% carbon in the form of lamp black or the like to the powder prior to hydrogen reduction, most all of the oxide content is removed. The addition of carbon also has the beneficial effect of minimizing decarbonization of the tool steel during subsequent processing. The carbon reacts with the oxides present to form CO and is given off as a gas from the powder. The substantial removal of the oxide impurity enables the process of the invention to produce a tool steel which can meet the properties of those previously existing in the art in its strength characteristics. As discussed hereinafter, the presence of small amounts of oxides uniformly distributed throughout the alloy steel product is tolerable, and in some instances appears to be beneficial.

It is important in the hydrogen reduction operation preceding cold pressing that the hydrogen reduced powder particles be handled under conditions to avoid imparting a Rockwell C hardness of more than about 55 to the powder charge employed in the subsequent cold pressing step. The recently hydrogen reduced powder in one procedure is desirably furnace cooled at a rate of less than 300 F. per hour and with this slow cooling will have a Rockwell C hardness not in excess of about 55. In an alternative procedure, the hydrogen reduced powder is removed from the hot zone of the furnace and permitted to air cool. The cooling from the elevated temperature is accomplished as rapidly as possible and at a more rapid rate than that practiced in the other procedure and in order to obtain a powder having a Rockwell C hardness of not more than about 55, the cooled powder is then subjected to a conventional over-tempering operation. Over-tempering is typically accomplished by heating the cool hydrogen reduced powder to around about 1200 F. The exact over-tempering temperature will depend upon the particular tool steel and may be lower or higher than 1200 F. The cooling from the elevated over-tempering temperature may be at any desired rate. Either of the foregoingprocedures will provide a'hydrogen reduced power that is susceptible to cold pressing. The critical aspect of the cooling of the hydrogen reduced powder is that the conditions be selected to assure a Rockwell C hardness of not more than about 55.

PRESSIN G A major advantage of the process of the invention over previous attempts to utilize powder metallurgical techniques in tool steel manufacture is elimination of hot pressing. Heretofore, hot pressing has been required in order to produce a billet having sufiicient green strength to maintain structural integrity upon further handling of the billet. With the process of the invention, adequate green strength may be obtained in the billet by cold pressing. The cold pressed body produced from the impact pulverized particles prossesses sufiicient structural integrity to enable it to be moved and placed in a sintering oven. Typically, the cold pressing is carried on at a pressure in excess of 60,000 p.s.i. and generally in the range of 90,000 to 100,000 p.s.i. Pressing may be accomplished either mechanically or isostatically. In one embodiment of the process of the invention, the cold pressing is achieved by explosive compaction. The theoretical density of the cold pressed billet is typically in excess of 55%, preferably in excess of 63%, and generally in the range of 5575% in the instance of mechanical and isostatic pressing. It has been demonstrated possible to cold press to a theoretical density of 86% where the powder employed has a Rockwell C hardness of 28. A preferred theoretical density range for cold pressed material produced by isostatic and mechanical pressing is 65-85%. With explosive compaction, which provides higher pressing pressures, the theoretical density will be considerably higher, typically in excess of 90% and desirably in the range of 98% to Cold pressing is accomplished at ambient or near ambient temperatures, with the pressing apparatus typically having no heating element.

SINTERING After completion of the cold pressed step of the invention, the compacted material which has already achieved a relatively good density and structural integrity is then subjected to a sintering operation. Typically, in the instance of isostatic and mechanical pressing, sintering will be accomplished in a furnace. In the instance of material formed by explosive compaction, sintering may be carried on in a furnace or by hot rolling of the material. Explosively compacted powder typically has a high density, for example 99% theoretical and with such high density material, it is possible to foster sintering by hot working without employing a furnace.

The sintering of the cold pressed billet and the like imparts a theoretical density of at least 85% to the tool steel. The minimum sintering temperature is that necessary to obtain the foregoing 85% theoretical density which is the minimum density required to hot work the billet without cracking. The sintering is carried on under conditions to minimize grain growth and phase separation of a degree insensitive to subsequent processing. That is, to say, an elevated sintering temperature which pro motes some limited grain growth and phase segregation may be tolerated if it is of a degree which may be corrected by subsequent working of the sintered material. It is sometimes feasible to accomplish a very high, acceptable theoretical density, for example densities of around 98% plus with sintering alone, and in such instances there will he sometimes no further need to hot roll, forge, or other Wise hot work the sintered material to gain a still higher density. Of course, if the conditions were such that the sintered material had acquired some objectionable grain growth and phase separation, it would still be necessary even though of theoretical density to subject the sintered billet to hot working to gain a homogeneous structure. However, hot working to obtain homogeneity in such instances will be much less severe than that of the prior art.

Sintering is preferably carried on in a hydrogen atmosphere or, if desired, in a carbonaceous atomsphere. A vacuum atmosphere is not recommended since the tool steel alloys there may be a tendency to vaporize some of the material such as, for example, chromium.

Sintering in the instance of isostatic and mechanical pressed billets may be accomplished typically over a general temperature range of about 2150 F. to about 2350 F. To those versed in the art, it is apparent that the sintering conditions may be varied to obtain a desired density of the billet, the sintering being, inter alia, a function of time, temperature, particle size, and mass of material being sintered. Minimum acceptable sintering is achieved by selecting the time and temperature necessary to obtain the minimum 85% theoretical density critical to further working. Maximum sintering is provided by selection of conditions of time and temperature which approach but avoid detrimental reactions such that subsequent processing typical to tool steel would be ineffective in providing characteristic tool steel properties. Examples of detrimental reactions tinclude melting of the material grain growth and phase segregation or separation so extensive that the material is insensitive to subsequent processing typical to tool steels.

It has been found, for example, in practice that for M2-S tool steel, more particularly discussed hereinafter, sintering is desirably accomplished within the temperature range of about 2225 F. to about 2330 F. for about 30 minutes. The temperature may be lowered to around 2200 F. if the time is extended to about one hour or in creased to around 2350 P. if the time is decreased to about 10 minutes.

The lower temperature limit for sintering in the instance of isostatic and mechanical pressed billets is established at about 2150 F. since lower temperatures would require too long of a sintering period for the process to be practical in obtaining the required material density. An upper limit of about 2350 F. is established as the maximum sintering temperture since with much higher temperatures eutectic formation is quite rapid and control of powder densification process becomes extremely difiicult. The foregoing temperature ranges may also be employed for explosively compacted tool steel powders which are subjected to a furnace sintering operation. In the instance where the sintering of the explosively compacted powder is achieved by working, for example by hot rolling, the temperature range is typically about 2200 F. to about 2000 F. with the working commencing at the former temperature and being terminated at the lower temperature. For any particular alloy composition, it is relatively easy to determine the preferred sintering temperature. The upper limit of the temperature range is that which induces grain growth and phase separation of a degree insensitive to subsequent processing. The low temperature possible for sintering of material is simply a matter of densification and is readily determined by experimentation.

It is found that the above conditions normally achieve a sintered density of at least theoretical. It is possible with a sintered density of 85% or above to elevate the density to around 99% of theoretical with relatively limited working of the sintered material. It is possible with judicious selection of conditions, particles sizes, etc. with the process of the invention to obtain densities up to 97-98% of theoretical in the sintering operation alone.

Another approach for enhancing densification is the liquid phase sintering procedure known to the art. In liquid phase sintering, a compound or metal is incorporated in the press compact which has a lower melting point than does the base metal. The additive forms a thin film of liquid metal over the base metal surfaces at the sintering temperature, thus increasing mass diffusion and densification. A particularly desirable liquid phase sintering agent which avoids introduction of a foreign element to the tool steel is molybdenum sulfide M08 The molybdenum sulfide may be conveniently added in an amount around 0.1 Weight percent with carbon (0.1 weight percent) to the hydrogen reduced steel powder prior to cold pressing. Another suitable liquid phase sintering agent is vanadium diboride, which is an exceptional hardener and serves to accelerate sintering.

Sintering time will vary considerably, and is determined primarily by the mass or weight of the material being sintered. The time must be suflicient for the desired densification to occur at the temperature chosen. An example of a typical sintering time at the temperature of 2300' F. is 30 minutes for a rectangular part of tool steel alloy of 1" x 1" x 4". The sintering time is also a function of the particle size of the material. Smaller particles typically require lower sintering temperature. Thus, the factor of particle size further enters into the consideration in choosing a suitable sintering temperature to achieve the desired densification.

HOT 'DEFORMATION Hot deformation refers to working a material under high temperature conditions. Examples include swaging, forging and hot rolling. Other procedures are well known in the art. This step in the process is primarily for densification. However, it also refines the structure by promoting homogeneity where needed. It has been found that hot deformation of the sintered product of this invention will achieve 100% theoretical densification. The working temperature range for the tool steels is typically in the range of 1800 F. to 2000 F. To highlight one of the particular advantages and unusual features of this instant invention, a 100% density homogeneous material can be frequently achieved with only a 40% reduction of the product. In contrast, in the prior art cast tool steel alloys, an extreme amount of hot working is required in order to achieve homogenization of the material. The prior art cast alloys normally require several hundred percent reduction in order to achieve sufficient homogenization. This extensive working requirement has prevented the use as tool steels, certain alloys which would be otherwise desirable materials. Thus, one of the particular advantages of this invention with its limited Working requirement is that it makes available alloy compositions which are not normally susceptible to the extensive hot working conditions required for homogenization in the prior art to provide an acceptable tool steel.

There are many alloys other than the normal tool steel alloys that are not cast because of the segregation problem. It can now be seen that in accordance with this invention one may atomize such alloys or water shot of such alloys, then applying the process of the invention produce a usable product since the segregation problem no longer exists.

HEAT TREATMENT The final step comprises heat treatment, a procedure conventional for tool steel alloys. The standard conditions known to the art include heating at 2200 F. for 10 minutes followed by an oil quench which in turn is followed by double temper at 1050 F. It has been found that the tool steels formed in accordance with this invention achieve a superior transformation from the austenite to martensite phase when the material is super cooled after the oil quench. Super cooling is not new to the art and is often used. Generally, the super cooling is accomplished by submerging the material in a cold bath, for example, liquid nitrogen which is at -320 F.

*It has been observed that a tool steel produced in accordance with the process of the invention can tolerate a higher percent oxide content than that normally found in conventional tool steel compositions. A tool steel composition known as M2-S tool steel typically is made up of by weight .9% carbon, 5% molybdenum, 4% chromium, 6% tungsten, 2% vanadium, .13% sulphur with the balance being iron. Normally an M2-S tool steel composition Will contain about .003 weight percent oxide. In one instance, examination of this same steel composition made in accordance with the process of the invention revealed an oxide content of approximately .06 weight percent, a remarkably higher oxide content. Despite this excessively high oxide percentage, the tool steel of the invention when subjected to machinability test performed successfully. It is known that with the process of the invention the tool steel alloy has an exceptional homogeneous structure and it is theorized that various oxides that may be present in the tool steel of the invention are evenly distributed throughout and may even serve in a beneficial role by reinforcing or supplmenting the cutting function of the carbides present in the tool steel alloy. There is evidence that such is the situation as in several instances tool steels produced in accordance with the process of the invention have exhibited relatively high oxide contents and despite relatively low carbon contents, the products have exhibited good machinability qualities.

It has not been possible to reheat the tool steels of the prior art made by casting for when such reheat treatment is attempted there occurs mass grain growth with a resultant loss of all material properties. Quite unexpectedly, it has been found that the tool steel alloys resulting from the process of the herein invention permit a reheat treatment without loss of material properties. Thus, if the desired phase transformation is missed during the first heat treatment cycle with the alloys of the instant invention, the heat treatment may be repeated to achieve the desired structure. This is because the grain growth of the alloys made in accordance with this invention is different from the grain growth of the prior art cast alloys in that it is less severe.

Another difference in properties sometimes found between a tool steel made in accordance with the invention and that of the prior art relates to hardness. Commercial cast and wrought tool steel such as M2-S tool steel has typically a Rockwell C hardness of 65-66 utilizing the conventional heat treatment. Through the use of a super cooled heat treatment, the hardness can be increased to 67.5. M2-S alloys processed in accordance with this invention have been found to have a Rockwell C hardness of 69 utilizing the super cooled heat treatment. This is completely unexpected and in fact, contrary to what has been believed possible in the prior art as will be shown. A sample of a MZ-S tool steel made in accord with this invention which achieved the Rockwell C hardness of 69 had a carbon content of only .57 weight percent. This is considerably less than the carbon content of usual M2-S tool steel which has a carbon content of .8-1.0 weight percent. As is well known, carbon is primarily present in the tool steel to give strength properties thereto. According to all available literature and information before this invention, it is theoretically impossible to achieve a Rockwell C hardness greater than 55 with a carbon content of only .57 weight percent. Thus, the achievement of a Rockwell C hardness even greater than the conventional tool steel, namely 69, is all the more 10 impressive in view of the fact that theoretically the Rockwell C hardness should not exceed 55.

Generally, in the instance of mechanically and isostatically pressed billets, a hot deformation step will be employed following sintering to further increase the theoretical density. Explosively compacted billets have characteristically high theoretical densities and in some applications will not need further working. In one embodiment of the process wherein liquid phase sintering is utilized, hot deforming may be dispensed with. The sintering temperatures generally are 50 F. to F. lower than for non-liquid sintering. Minor alloy elements are added prior to sintering. Any element that has a melting point lower than that of the alloy and will form an alloy with the basic material can be used for liquid phase sintering. An example is the metal halides which would be added in the amount of 5 weight percent or less. Additionally, nickel, copper, cobalt in the amounts of about .25 Weight percent all serve as liquid phase sintering agents. The particular particle size of these additives is not critical; they are normally added in the form of powder in the micron size range. It is possible to get as-sintered densities of 97% and greater utilizing liquid phase sintering.

Various ones of known tool steels may be used in the process of the invention including those commonly designated water hardening, cold work, hot work, high speed and stainless. Additionally, as mentioned before, the limited working requirements of the process of the invention permits the use as tool steels alloys which are normally not available for this use.

Following are examples of process and product in accordance with the invention.

EXAMPLE I Scrap M2-S tool steel from machine turnings is subject to the Cold Stream process of Pat. No. 3,184,169 to obtain a powder -325 mesh size fraction. Carbon in the form of lamp black is added to the powder in an amount of .25 Weight percent. The powder is then placed in an oven under a hydrogen atmosphere at 1800 F. The residence time in the oven is 1 hour during which reduction of the oxide occurs. The powder is then removed from the oven and mechanically cold pressed at 100,000 p.s.i. to form a rectangular sample 1" x 1" x 4". The cold pressed sample is placed in an oven where sintering occurs at 2200" F. for 1 hour. The sample is then hot worked by rolling at 1800 F. During the rolling a 15% reduction is utilized at each step to achieve a total reduction of about 50%. The rolling is then followed by the heat treatment utilizing a super cooling technique. More precisely, the sample is initially subjected to a temperature of 2200 F. for 10 minutes, followed by an oil quench. After the oil quench the sample is then subjected to a liquid nitrogen bath which is at 320 F. The resultant sample has a density of 100%. The Rockwell C hardness is 69, while the carbon content is determined to be .57% by utilizing conductimetric technique. Machinability tests indicate that the sample out performs cast and wrought tool steel of the standard MZ-S composition.

It is noted that the product made in accordance with the invention in this Example I has a significantly lower carbon content than the original M2-S tool steel alloy. As noted, originally M2-S alloy typically has a .9% carbon content whereas the product of this example has only a .57% carbon content. The loss of carbon is believed to result from its combination with the oxides present to form CO, leaving the material as a gas. Thus, even though additional carbon is added for cleaning purposes, it is inadequate to prevent some decarbonization of the tool alloy. If desired, additional carbon could be added to maintain any desired carbon level. In this example, the loss of carbon did not prove objectionable since the resultant product exhibits improved properties over the more conventional material having the higher carbon content. This example demonstrates that a new form of tool steel having unexpected properties results from performing the process of the invention.

Scrap M-30 tool steel and scrap H-l2 tool steel proccessed in accordance with the procedure of this Example I provide quality products.

EXAMPLE II Atomized M-lOW tool steel is impact pulverized employing the process of US. Pat. No. 3,184,169 to obtain a powdered 100 mesh size fraction. The fraction is hydrogen reduced at 1800 F. in an oven after adding to the powder .50 weight percent in the form of lamp black. The residence time in the oven is 1 hour. After reduction the powder is ball milled and screened to give a -100 mesh size fraction which is then explosively compacted into a A inch diameter x 4 foot long test bar. The explosively compacted powdered billet has a theoretical density of 99%. Following compaction, the explosively compacted rod is sintered in hydrogen for 30 minutes at 2282 F. to attain a 100% theoretical density. The sintered product is then heat treated in accordance with the procedure of Example I and gives a product having a Rockwell C hardness of 66. The explosively compacted tool material exhibits a performance substantially comparable to that of its pressed and sintered counterpart.

Scrap T-l tool steel may be processed in accordance with Example II to obtain a quality tool steel product.

EXAMPLE III The billet prepared in this example is generally like that of preceding Example II except that sintering is achieved by hot rolling rather than a sinter furnace. The recommended rolling temperature is between 2200 F. and 2000 F. The product produced by hot rolling is comparable to that obtained by the sinter furnace approach of Example 11.

EXAMPLE IV This example demonstrates the use of liquid phase sintering in the practice of the process of the invention. Atomized M2-S tool steel is impact pulverized to obtain a powder l mesh size fraction. The powder is hydrogen reduced in an oven of the temperature of 1750 F. for 2 hours. MoS (0.1 weight percent of the powder) is added with carbon (0.1 weight percent powder) to the hydrogen reduced M2S tool steel powder. The powder is isostatically cold pressed at around 100,000 p.s.i. to form several test bars. Some of the test bars are sintered for 30 minutes in a furnace at 2270 F. and other test bars sintered at 2290 F. for 30 minutes. The lower temperature sintered bars show theoretical densities of 77 and 84 while the higher temperature sintered bars exhibit theoretical densities of 87 to 100%. The sintered structures appear to be relatively clean and show no indications of sulfides. Chemical analysis detects only .08 weight percent sulphur which is below the acceptable specification limit. Some spotty areas of eutectic are noted to exist throughout the structures. Hot working breaks the eutectic up and distributes it so that it is dissolved in the subsequent heat treatment. The liquid phase sintering approach is considered a success as is evident in heat treatment and machinability results from the samples.

Vanadium dibromide may be substituted for molybdenum sulfide M08 as the liquid phase sintering agent. Liquid phase sintering may be performed with various ones of the tool steels including the shock resisting alloy S5, the cold Work alloy 01 and the tungsten base alloy T3.

The presently preferred embodiment of the process of the invention comprises blending the impact pulverized powder having an 8 micron average particle size with an 0.25 weight percent carbon addition and hydrogen reducing at 1850 F. for 1 hour followed by a slow furnace cooling. The pulverized powder may be prepared from scrap or virgin tool steel alloy or blend of the two. The hydrogen reduced powder is then ball milled for 30 minutes and screened through a -325 mesh prior to pressing. The powder is then isostatically pressed at 100,000 p.s.i. to give a theoretical density somewhat larger than 65%. The pressed form is then sintered in a carbon pot furnace to at least minimum theoretical density. The sintering is conducted in a hydrogen atmosphere. Heating is rapid to 2012 F. with a 15 minute hold at preheat temperature, and again rapid to the sintering temperature, 2300 F., with a 30 minute hold at that temperature followed by a furnace cooling. The sintered forms are then reduced to around 40 to 50% by either upset forging or swaging. This mechanical reduction is carried on in an endothermic atmosphere wherever possible to minimize carbonization. The heating between the mechanical reductions will vary with the mass of the part but will average around 5 minutes. The densified form or billet is then subjected to a stress relief by annealing for 1 hour at 1600 F. in an endothermic or neutral atmosphere immediately after the foregoing hot working. The cooling rate from the annealing temperature is slow, approximately 50 F. per hour. Following stress relief, the tool blanks is then subjected to a conventional heat treatment, for example, in a salt bath employing standard commercial schedule or alternatively an oil-solvent liquid nitrogen quench may be employed. The powder metallurgical tool steels produced in accordance with the process of the invention have greater alloy homogeneity and conse quently improved dimensional stability and performance characteristics as compared with the currently available cast and forged tool steels.

While several embodiments of the invention have been illustrated and described, it will be understood that the invention should not be construed as being limited thereto, but only to the lawful scope of the appended claims.

We claim:

1. Method of making an alloyed steel of superior alloy homogeneity which comprises cold pressing to a coherent compact mass, a powder of irregularly-shaped particles, of an average particle size within the range of 2 to 60 microns, of a substantially oxide-free homogeneous alloy of the steel, said particles being produced by impact pulverization of a pro-alloyed steel, then sintering said mass.

2. A method in accordance with claim 1 inwhich the impact pulverization of said pre-alloyed steel is performed on material selected from the group consisting of water shot, atomized and scrap tool steel and mixtures thereof.

3. Method according to claim 1 in which the oxide-free quality of the particles is attained by hydrogen reduction of said particles prior to the said cold pressing.

4. In a powder metallurgy method of producing a tool steel of superior alloy homogeneity including the steps of cold pressing a powder charge to a billet form, and sintering the billet, the improvement comprising: utilizing as the charge in the cold pressing step, irregularly-shaped hydrogen-reduced tool steel powder particles of a homogeneous composition having a size not greater than mesh and with an average particle size in the range of 2 to 60 microns produced by impact pulverization of a pre-alloyed material, said impact pulverization effecting a fracturing of the pro-alloyed material into the irregularly-shaped particles.

5. A method in accordance with claim 4 wherein the average particle size is in the range of 6 to 14 microns andhthe maximum particle size is not greater than 325 mes 6. A method in accordance with claim 4 wherein the distribution curve of the particle sizes of the powder approximates a Gaussian ratio.

7. In a method of producing an improved tool steel having a superior alloy homogeneity, the steps comprising:

providing a charge of irregularly-shaped tool steel powder particles of a homogeneous composition produced by impact pulverization of a pre-alloyed material, said powder having an average particle size in the range of 2 to 60 microns; exposing the powder particles to a hydrogen reducing atmosphere at an elevated reducing temperature in the range of 1600 F. to 1900 F. to reduce the oxygen content and handling the reduced powder particles preceding the subsequent cold pressing step under conditions selected to avoid imparting a Rockwell C hardness of more than about 55;

cold pressing the reduced powder particles to a billet form having green strength adequate to maintain structural integrity upon further handling of the billet; and

sintering said cold pressed billet to impart a theoretical density of at least 85% to the tool steel, said sintering being carried on in a furnace at a temperature in the range of about 2150 F. to about 2350 F.

8. A method in accordance with claim 7 wherein the irregularly-shaped powder particles are prepared by impact pulverizing of pre-alloyed material selected from group consisting of water shot, atomized, and scrap tool steel and mixtures thereof.

9. A method in accordance with claim 7 wherein the distribution curve of the particle sizes of the powder approximates a Gaussian ratio.

10. A method in accordance with claim 7 wherein the average particle size is in the range of 6 to 14 microns.

11. A method in accordance with claim 7 wherein decarbonization is minimized by incorporation of a small amount of carbonaceous substance in the powder charge.

12. A method in accordance with claim 11 wherein the carbon is added in an amount of 0.1 to 1.0 weight percent of the powder charge.

13. A method in accordance with claim 7 wherein the hydrogen reduction is carried on at a temperature in the range of 1750 F. to 1875 F.

14. A method in accordance with claim 7 wherein the powder from the hydrogen reduction is furnace cooled from the elevated reducing temperature at a rate slower than about 300 F./hour.

15. A method in accordance with claim 7 wherein the cold pressing is carried on to obtain a minimum theoretical density of 63%.

16. A method in accordance with claim 7 wherein the cold pressing is accomplished by explosive compaction.

17. A process in accordance with claim 16 wherein the compacted billet is subjected to a furnace sintering.

18. A method in accordance with claim 16 wherein sintering of the compacted billet is accomplished by hot deforming to a theoretical density of at least 99%.

19. A method in accordance with claim 7 wherein a liquid-phase sintering is employed.

20. A method in accordance with claim 7 wherein the sintered billet is subjected to a hot deforming operation to obtain a substantially 100% theoretical density material.

21. A method in accordance with claim 20 wherein there is a reduction of less than about 50% during the hot deforming operation.

22. A method in accordance with claim 7 wherein the powder from the hydrogen reduction is permitted to air cool from the elevated reducing temperature and, thereafter, subjected to an over-tempering operation to provide a Rockwell C hardness of not more than about 55.

23. Method of making an alloyed steel of superior alloy homogeneity which comprises cold pressing to a coherent compact mass, a powder of irregularly-shaped particles of an average particle size within the range of 2 to 60 microns of a substantially oxide-free homogeneous alloy of the steel, said particles being produced by impact pulverization of a pro-alloyed stel with the distribution curve of the particle sizes of the powder approximating a Gaussian ratio, then sintering said compact mass.

24. Method of making an alloyed steel of superior alloy homogeneity which comprises cold pressing to a coherent compact mass, a powder of irregularly-shaped particles of an average particle size within the range of 6 to 14 microns of a substantially oxide-free homogeneous alloy of the steel, said particles being produced by impact pulverization of a pre-alloyed steel with the distribution curve of the particle sizes of the powder approximating a Gaussian ratio, then sintering said compact mass.

25. In a powder metallurgy method of producing a tool steel of superior alloy homogeneity including the steps of cold pressing a powder charge to a coherent compact mass, and sintering the compact mass, the improvement comprising:

utilizing as the charge in the cold pressing step irregularly-shaped, hydrogen-reduced tool steel particles of a homogeneous composition having an average particle size in the range of 2 to 60 microns produced by impact pulverization of a pre-alloyed material "with the distribution curve of the particle sizes of the powder approximating a Gaussian ratio. 26. In a powder metallurgy method of producing a tool steel of superior alloy homogeneity including the steps of cold pressing a powder charge to a coherent compact mass, and sintering the compact mass, the improvement comprising:

utilizing as the charge in the cold pressing step irregularly-shaped, hydrogen-reduced tool steel particles of a homogeneous composition having an average particle size in the range of 6 to 14 microns produced by impact pulverization of a pre-alloyed material with the distribution curve of the particle sizes of the powder approximating a Gaussian ratio. 27. A process in accordance with claim 7, wherein the cold pressing is done mechanically.

28. A process in accordance with claim 7, wherein the cold pressing is done isostatically.

29. In a method of producing an improved tool steel having a superior alloy homogeneity, the steps comprismg:

providing a charge of irregularly-shaped tool steel powder particles of a homogeneous composition produced by impact pulverization of a pre-alloyed material, said powder having an average particle size in the range of 2 to 60 microns; exposing the powder particles to a hydrogen reducing atmosphere at an elevated reducing temperature in the range of 1600 F. to 1900 F. to reduce the oxygen content and handling the reduced powder particles preceding the subsequent cold pressing step under conditions selected to avoid imparting a Rockwell C hardness of more than about 5 5;

cold pressing the reduced powder particles by explosive compaction to a billet form having green strength adequate to maintain structural integrity upon further handling of the billet; and

sintering said explosively compacted billet by hot rolling in the range of 2000 F. to 2200 F. to impart a theoretical density of at least to the tool steel.

30. A method in accordance with claim 29 wherein the distribution curve of the particle sizes of the powder approximates a Gaussian ratio.

31. A method in accordance with claim 29 wherein the average particle size is in the range of 6 to 14 microns.

32. A method in accordance with claim 29 wherein decarbonization is minimized by incorporation of a small amount of carbonaceous substance in the powder charge.

33. A method in accordance with claim 29 wherein the carbon is added in an amount of 0.1 to 1.0 weight percent of the powder charge.

34. A method in accordance with claim 29 wherein the hydrogen reduction is carried on at a temperature in the range of 1750 F. to 1875 F.

35. A method in accordance with claim 29 wherein the powder from the hydrogen reduction is furnace cooled 15 16 from the elevated reducing temperature at a rate slower OTHER REFERENCES than about R/hom" Jones: Fundamental Principles of Powder Metallurgy,

Edward Arnold, 1960, pp. 190-191, 224427, 344-351. References Cited UNITED STATES PATENTS 5 LELAND A. SEBASTIAN, Primary Examiner 3,184,169 3/1963 Friedman et a1 241-40 SCHAFER, Assistant Examiner FOREIGN PATENTS US. 01. X.R. 781,083 8/1957 Great Britain 75226 10 75 200, 214, 223, 224, 226, 227; 148126 Egg? UNITED STATES PATENT OFFICE CER'HFICATE OF CORRECTICN Patent No. 3,7Mh993 Dated July 10, 1973 Inventofls) Robert E. Matt and Alan V. Levy It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 67, "or" should be of-.

Column 5, after line 43, insert about one hour. The range of temperature may extend be- Column 5, delete line H8 in its entirety, i.e. "about one hour.

The range of temperature may extend be" Column 6, line 17, "power" should be powder-.

Column 9, line 29, "supplmenting" should be supplementing-.

Column 13, line 72, "stel" should be steel-.

Signed and ficalcd this twentieth D y of January 1 6 [SEAL] Arrest:

RUTH C. MASON C. MARSHALL DANN Arresting Offiter Commissioner ofPaIents and Trademarks I I UNITED STATES PATENT OFFICE CERTEFICATE OF CGRRECTION Patent No. 3,7 l l,993 Dated July 10, 1973 Inventor(s) Robert E. Matt and Alan V. Levy It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 67, "or" should be -of'-.

Column 5, after line 13, insert about one hour. The range of temperature may extend be- Column 5, delete line 18 in its entirety, i.e. "about one hour.

The range of temperature may extend be-" Column 6, line 17, "power" should be --powder.

. Column 9 line 29, "supplmenting" should be supplementing-.

Column 13, line 72, "stel" should be -steel-.

0 Signed and Scaled this twentieth D 0f January 1976 [SEAL] Attest: O

RUTH C. MASON C. MARSHALL DANN Attesting Officer Commissioner oflarents and Trademarks 

